The present invention relates to a method and an apparatus for detecting and counteracting a deformation of the stock discharge gap in a headbox of a paper machine during its operation. The headbox includes a roof member and an apron beam member which cooperate to define the stock discharge gap therebetween.
Headboxes are included in the wet end of a paper machine and are used to distribute the stock uniformly over the width of the wire and for controlling the rate of discharge so that the stock is deposited onto the wire at equal velocities and in the same direction along the whole width of the wire. Demands for higher manufacturing velocities in paper machines cause increased problems with producing a paper web of satisfactory quality in respect of, inter alia, a uniform basis weight. Therefore, it is of great importance to keep the shape of the slice opening constant and under careful control so that the discharged stock Jet will be uniform with respect to velocity and thickness and so that as a consequence thereof optimum dry weight and moisture profiles of the paper web are obtained.
A number of parameters may influence the shape of the discharge gap. For example, the pressure of the stock in the headbox can cause a deflection in the central portion of the lips and result in an increased basis weight in this area. In accordance with U.S. Pat. No. 3,468,756 (Villa), such a deflection is counteracted by heating the support beam directly beneath the lower lip and/or by cooling the lower portion of the frame included in the apron beam in such a manner that the deflection of the lower lip is counteracted. This correction is based on a theoretical calculation of the deflection, and the patent fails to provide any apparatus or method by which the actual deflection of the lip may be determined while the machine is in operation. U.S. Pat. No. 3,994,773 (Wolf et al.) discloses an apparatus for indirectly detecting the deflection of the upper lip of a headbox by measuring a displacement between measuring elements mounted on top of the lip. The measuring method disclosed in this patent provides an approximate result as to the deflection, which is then counteracted by adjusting spindles supporting the upper side of the lip. Similarly, U.S. Pat. No. 4,342,619 (Gladh) discloses an apparatus for detecting the deflection of a profile bar mounted on the upper slice lip. However, neither Wolf et al. nor Gladh provides an apparatus for detecting or correcting relative displacements between both lips.
An obJect of the present invention is to provide more accurate detection and counteraction of a deformation in the shape of the stock discharge gap than that which has been achievable previously.